Device for removing material from a workpiece by means of spark erosion

ABSTRACT

A spark erosion device including a storage capacitor which is periodically discharged across the gap between the electrode and workpiece. The improvement concerns means for limiting the duration of the arc discharge phase of the operation comprising, a source of constant current for charging the capacitor, means for disconnecting the current source from the capacitor when the gap breakdown voltage is reached, and the addition of an inductor in the capacitor discharge circuit to provide a resonant effect to limit the arc time across the gap.

United States Patent Inventors Cornelis van Osenbruggen;

Gerrit Luimes; Ate van Diik, all of 1 Emmasingel, Eindhoven, NetherlandsAppl. No. 20,874 Filed. Mar. 27, 1970 Patented Dec. 14, 1971 AssigneeNorth American Philips Company Inc.

New York, N.Y. Priority Oct. 16, 1965 Netherlands Continuation oiapplication Ser. No. 587,326, Oct. 17, 1966. This application Mar. 27,1970, Ser. No. 20,874

DEVICE FOR REMOVING MATERIAL FROM A WORKPIECE BY MEANS OF SPARK EROSION15 Claims, 4 Drawing Figs.

U.S. CL... 2l9/69C Int. Cl B23k 9/16 Field of Search 219/69 E, 69 C, 69P References Cited UNITED STATES PATENTS 2,773,168 12/1956 Williams2l9/69C X 3,033,971 5/1962 Pfau 219/69 C 3,259,795 7/1966 Schicrholt219/69 C X Primary Examiner-J. V. Truhe Assistant Examiner-Hugh D.Jaeger Attorney-F rank R. Trifari ABSTRACT: A spark erosion deviceincluding a storage capacitor which is periodically discharged acrossthe gap between the electrode and workpiece. The improvement concernsmeans for limiting the duration of the arc discharge phase of theoperation comprising, a source of constant current for charging thecapacitor, means for disconnecting the current source from the capacitorwhen the gap breakdown voltage is reached, and the addition of aninductor in the capacitor discharge circuit to provide a resonant effectto limit the are time across the gap.

PATENTEU DEC 1 4 am SHEET 1 OF 2 FIG.1

CORNELIS VAN os s rfig l l 25m Lu MES IJK AGENT PATENTEU DEB] 41971SHEET 2 0F 2 7 FIG-.4

I N VENTORS CORNELIS VAN OSENBRUGGEN 666M a workpiece by means of sparkerosion.

Such a device is known. In-the known device a capacitor is arranged inparallel with the working area, i.e. the space between the workpiece andthe working electrode, said capacitor being charged through a resistorfrom the voltage source. When the capacitor has charged to a givenvoltage it is discharged through the dielectric medium between theworkpiece and the electrode. The spark energy is capable of removingmaterial from the workpiece.

The known device has several disadvantages.

In the first place the charging period of the capacitor is limited bythe value of the charging resistor. This value must be higher than acritical resistance to prevent an arc discharge in the working space.Furthermore, in the case of a short circuit between the workpiece andthe electrode, a maximum current occurs which is likewise determined bythe charging resistor. This current must not be excessive for electrodeshaving small working surfaces in order to avoid deterioration of theelectrodes. The charging resistance which, necessarily, is high, resultsin an unnecessarily low recurrence frequency of the spark discharges andhence in a low rate of chipping.

In the second place a long charging period is often undesirable sincethe presence of impurities, for example, metal particles which have beenreleased through the spark erosion which is in progress, may give riseto premature formation of a spark. The said particles are believed toform bridges which affect the normal course of the equipotential linesbetween the electrode and the workpiece, and thereby cause anaccelerated premature breakdown. The buildup of the electrical fieldbetween the particles and the electrode takes a finite time, forexample, several hundredths of a microsecond. A short charging period ofthe capacitor can ensure that this buildup period is not reached.

In the third place, the substantially total discharge of the capacitorproceeds comparatively slowly. During the first part of the dischargeperiod, the discharge has the character of a spark-The spark ischaracterized by a comparatively high voltage and a comparatively small,but very rapidly increasing current, and hence a high di/dt. During thesecond part of the discharge period, the discharge has the character ofan arc. The arc is characterized by a comparatively low voltage and acurrent which is large, but increases only slightly in time, and hence alow di/dt.

The spark and are phases of the capacitor discharge naturally merge intoeach other progressively.

Investigation of the phenomena has shown that the arc phase of thecapacitor discharge is of little use, and is even harmful for precisiontreatments in the case of chipping by means of spark erosion. Thesurface condition of both the workpiece and the electrode is affectedadversely due to thermal effects of the arc discharge. It appears fromexperiments that the wear ratio, that is the ratio between the amount ofmaterial removed from the workpiece and the amount removed from theelectrode, gets worse as the arc discharge lasts longer.

Accordingly, an object of the invention is to prevent an excessive shortcircuit current between workpiece and electrode as well as to reduce thecharging period and to minimize or eliminate the arc discharge phase ofthe electric discharge operation. I

To this end, the invention is characterized in that the capacitor ischarged through a charging circuit having current-limiting properties,means being provided which, at the instant of breakdown, cause thevoltage across the discharge space to decrease below the extinctionvoltage of the spark in a period of time which is shorter than the timeperiod corresponding to the discharge time constant of the circuitconstituted by the capacitor and the spark gap.

Underlying the invention is the concept that prolonged charging anddischarging periods cause other undesirable phenomena, that is to say, adischarge which is premature and hence relatively ineffective in thecase of a long charging period of the capacitor connected across theworking space, and an unfavorable wear ratio and harmful attack of theworkpiece surface in the, case of an unduly long discharging time ofsaid capacitor. Limitation of the total period in which a voltage existsbetween electrode and workpiece is also desirable in cases where thedielectric medium between the electrode and the workpiece exhibitselectrochemical effects such as, for example, electrolysis.

In one advantageous embodiment the capacitor is charged from a source ofconstant current, the supply of current being interrupted when thedesired voltage on the capacitor is reached, the charging circuit of theworkingarea including a resonant circuit having an inductor.

In order that the invention may be readily carried into effect, it willnow be described in detail, by way of example, with reference to theaccompanying diagrammatic drawings, in which:

FIG. I shows a diagram of a first device according to the invention;

FIG. 2 shows diagrams to clarify the operation of the device of FIG. 1;

FIG. 3 shows a diagram of a second device according to the invention,and

FIG. 4 shows diagrams to clarify the operation of the device of FIG. 3.

In the'circuit of FIG. I a capacitor 12 is charged from a direct voltagesource 1 through a pentode 2. A cathode resistor 9 serves as a negativefeedback resistor. The current flowing through pentode 2, and hence thecharging current of capacitor 12, is maintained constant. When capacitorI2, and hence also a capacitor 15 placed between a working electrode 14and a workpiece 13, has charged to a given value, the charging currentsuddenly stops. The charging circuit is designed so that the current isinterrupted at the breakdown voltage of the working area. The capacitor15 may consist of the capacitance of the spark gap defined by electrode14 and workpiece l3.

The abrupt suppression of the current supply to capacitor 12 takes placeas follows. At the beginning of the charge period, at the instant t=t,,in the diagram of FIG. 2, the potential of the junction point 21 ofcapacitor 12 and cathode resistor 9 is much lower than that of the anodeof pentode 2. A diode 5 connected between point 21 and the control gridof pentode 2, is conducting and the voltage drop across resistor 8, inseries therewith, is high enough to keep a diode 6 cut off. In fact, apentode 3, which is connected in series with a resistor 7 and a gaseousdischarge tube 4 to the voltage source I, is substantially cut off sincethe voltage drop across resistors 10 and 11 is negligible, so that thecontrol grid of the tube 3 is negatively adjusted relative to itscathode. The voltage across capacitor 12, and hence the voltage dropacross the resistors 10 and 11, increases due to the charging process.The tube 3 begins to conduct. The voltage drop across anode resistor 7,which was negligible at the beginning of the charging time, increases.Finally, the anode potential of pentode 3 becomes lower than thepotential ofthe control grid of pentode 2. The diode 6, which isconnected between the control grid of pentode 2 and the anode of pentode3, now conducts and the control grid assumes the low potential of theanode of pentode 3, thereby causing diode 5 and the pentode 2 to cutoff. The charging of capacitor 12 stops abruptly.

A spark discharge occurs in the medium between the working electrode 14and the workpiece 13 between the instants I, and 1, (FIG. 2). During thespark discharge the voltage across the gap decreases very rapidly to thearc voltage ('g'[ is of the order of at most a few tenths of amicrosecond). Due to the discharge of capacitor 15, a high currentmomentarily occurs in the discharge circuit. For maintaining an arcdischarge, it is necessary to supply a current from without, that is tosay from capacitor 12. However, an' abrupt supply of current isprevented by an inductor 16. The are produced is extinguished since thevoltage across the working area has decreased below the ignitionvoltage. Capacitor 15 is now charged again. The coaction of inductor 16and capacitor 15 causes a sinusoidal voltage waveform across the workingarea having a frequency which is substantially determined by the valuesof inductor 16 and capacitor 15. A resistor 17 in series with inductor16 and capacitor 15 ensuresthat the alternating voltage is rapidlydamped.

The voltage across capacitor 12, which has decreased slightly due to thecharge supplied to the capacitor 15, is recharged to its initial valuefrom the voltage source 1.

A subsequent breakdown between the electrode 14 and the workpiece 13occurs at the instant t=t In fact, the spark gap medium needs some timeto recover from the previous breakdown between the instants t, and t Inone embodiment the pentodes 2 and 3 were of the type E 130 L, the diodes5 and 6 were of the type 0A 90 and BY 100, respectively. The resistors7, 8, l8 and 19 were 10 kQ,0 100 M1220!) and 22k!) respectively. Theresistors 9, 10, and 11 were at most lkfl, 22k!) and SOkQ, respectively.Capacitor 12 was 180 pf., inductor 16 was 25 H, resistor 17 was 470. andcapacitor had a value of 60 pf. The time t t, was of the order of 30nanoseconds. The time t t was l.2 microseconds.

In FIG. 2, the voltage across, and the current through, the working areaare plotted as a function of time.

The charge current of capacitor 12 is controlled by varying the resistor9. The voltage across capacitor 12, which is matched to the breakdownvoltage of the space between the workpiece 13 and the working electrode14, is adjusted by connecting the control grid of pentode 3 to avariable tapping on resistor 11.

It will be evident that the current from the voltage source 1 isdetermined by the cathode resistor 9 of pentode 2 and is not influencedby any short circuit of the working area.

A circuit is shown in FIG. 3 which has a very short charge period of thecapacitor connected across the working area. This circuit is thereforeespecially suitable in the event that unwanted electrochemical effectsmay occur in the medium.

Short pulses originating from a pulse source 40 are fed to the controlgrid of an amplifying tube 30. The pulses appearing at the anode of tube30 are transmitted in turn to the control grid of a pentode 31, which isconnected as a cathode follower. The cathode follower 31 controls anoutput tube 33. The output circuit of output tube 33 includes aparasitic capacitance 62 formed by the working electrode 34 and theworkpiece 35. A discharge capacitor 61 is connected in parallel withsaid capacitance. It is also possible to use only the anode capacitanceof output tube 33 as the discharge capacitor. The discharge capacitor 61is charged in a very short time (from 0.1 to l 11sec), whereafter aspark discharge takes place in the medium between a working electrode 34and a workpiece 25. The voltage across the working area rapidlydecreases to the arc voltage. The arc is suppressed very rapidly so thatthe total duration of the charging and discharging processes is shorterthan the duration of the pulses derived from pulse source 40.

The suppression of the arc takes place as follows: The voltage drop fromspark voltage to are voltage is transmitted through a capacitor 36 to'the commonly connected control grids of a double tetrode 32, whichbecome positive relative to the cathode potential due to the voltagedropv and the tetrode becomes conducting. One half of dual tetrode 32 isconnected in parallel with a cathode resistor 44 of the cathode follower31, and the other half is of the dual tetrode included in the controlcircuit of the cathode follower 31.

The effect of the tetrode 32 becoming conducting is that the controlgrids of the tubes 31 and 33 assume a more negative potential. Theoutput tube 33 and the tube 31 cut ofi, although the output pulse fromthe source 40 is still present. The voltage difference between theworking electrode 34 and the workpiece 35 disappears almost completely.

in one embodiment tube 30 was of the type 001:. 02/5, tube 31 was of thetype EL 86, tube 32 was of the type 005 02/5 and tube 33 was of the typeEL 500. The resistors 41, 43, 44, 45, 46, 48, 49 and 50 were 33 k0, k0,2.2 k0, 4.7 k0, 390 O, 47 k0, 100 k!) and 3,3 k0, respectively. Theresistors 51, 52 and 60 were 680 Q, 27 0 and respectively. Capacitor 42was 10,000 pf. capacitor 47 was 0.27 pf. and capacitor 36 was 30pf.Capacitor varied between 100 pf. 20,000 pf. and capacitor 62 was 50 pf.

Pulses of 2 psec. duration having a recurrence frequency of 100 kc./sec.were derived from the voltage source 40. The total charge period ofcapacitor 62 was 0.5 usec. The spark period t -t, was 100 nanoseconds.The greatly reduced arc period t t was 200 nanoseconds and the durationu-t, was 10 psec. The maximum current during the discharge period was 30amps.

In order to suppress a reverse pulse, it is possible to arrange a diodein parallel with capacitor 62. A series resistor of several ohms placedbetween capacitor 61 and capacitor 62 may serve the same purpose.

What is claimed is:

1. A device for electrically eroding material from a workpiece by meansof electric spark discharges produced across a spark gap work areadefined by the workpiece and an adjacent electrode comprising, a sourceof electric energy, a capacitor, a discharge circuit connecting saidcapacitor to said electrode and workpiece, a charging circuit havingcurrent-limiting properties that are independent of the spark gap andconnecting said capacitor to said energy source, and means for causing,at the instant of gap breakdown, the voltage across the work dischargearea to decrease to a level below the spark extinction voltage of thespark gap work area in a period of time which is shorter than the timeperiod corresponding to the time constant of the discharge circuitconstituted by the capacitor and the spark gap.

2. A device as claimed in claim 1 wherein said energy source comprises asource of constant current, means responsive to the capacitor voltagefor interrupting the supply of current to the capacitor at a givenvoltage across the capacitor, and a charging circuit for the gap workarea including an inductor that forms a resonant circuit therewith.

3. A device as claimed in claim 1 further comprising, a first activecontrol element connected between the energy source and the capacitor, asecond active control element connected in parallel with the energysource, a first rectifier arranged between a control electrode of thefirst active element and the capacitor, a second rectifier connected inseries with the second active element and connected to the controlelectrode of the first active element, and means for deriving a controlvoltage for the second active element from the voltage across thecapacitor.

4. A device as claimed in claim 1 further comprising, control meansconnected between the energy source and the capacitor, an amplifyingelement which forms part of the transmission path between the energysource and the capacitor, means for coupling the output of saidamplifying element to said control means to control the conditionthereof, and means for coupling the voltage across the capacitor to thecontrol electrode of the amplifying element so as to produce a cutoficondition in said means thereby to cut off the voltage from the sourcewhen the voltage across the capacitor decreases.

5. A device as claimed in claim 4 wherein said control means includes acathode follower device connected in the transmission path and having acathode resistor, and means connecting the amplifying element inparallel with the cathode resistor of said cathode follower device.

6. A device as claimed in claim 1 further comprising means coupled tosaid capacitor and responsive to the capacitor voltage for interruptingthe supply of charging current to the capacitor when the capacitor ischarged up to the breakdown voltage of the gap work area.

7. A device as claimed in claim 1 wherein said energy source and saidcharge circuit together from a source of constant current for saidcapacitor and the spark gap work area which limits the current to aconstant value during the whole spark erosion process.

8. A device as claimed in claim 1 further comprising, an inductorconnected in series with the gap work area, and means connecting theseries combination of the inductor and the gap work area in parallelwith said capacitor across the energy source.

9. Spark erosion apparatus for removing material from a workpiececomprising, an electrode confronting said workpiece to define a sparkdischarge gap, a source of electric energy, a capacitor, a chargecircuit having current-limiting properties that are independent of thespark gap, means including said charge circuit for coupling saidcapacitor and said spark gap to said energy source, a discharge circuitconnecting said capacitor to said electrode and workpiece, and meanscontrolled by the capacitor voltage for periodically interrupting theflow of charge current to said capacitor whenever the gap voltagereaches the gap breakdown voltage.

10. Apparatus as claimed in claim 9 further comprising a secondcapacitor directly connected across said electrode and workpiece, saidsecond capacitor and said spark gap being chosen to have a dischargetime constant that is shorter than the discharge time constant of thefirst capacitor and said spark gap.

11. Apparatus as claimed in claim 10 further comprising an inductorconnected in the charge circuit of said second capacitor and in thedischarge circuit of the first capacitor.

12. Apparatus as claimed in claim 11 wherein said energy sourcecomprises a source of constant current, and said interrupting meanscomprises a control element serially connected in the charge circuits ofsaid first and second capacitors.

13. Apparatus as claimed in claim 9 further comprising a nonsaturatinginductor connected in the discharge circuit of said capacitor forlimiting the flow of discharge current from the capacitor to the sparkgap during the breakdown periods of the gap.

14. Apparatus as claimed in claim 9 wherein said interrupting meanscomprises a control element serially connected in the charge circuits ofsaid capacitor and the spark discharge gap.

-15. Apparatus as claimed in claim 9 wherein said energy source and saidcharge circuit together form a source of constant current for saidcapacitor and the spark discharge gap.

1. A device for electrically eroding material from a workpiece by meansof electric spark discharges produced across a spark gap work areadefined by the workpiece and an adjacent electrode comprising, a sourceof electric energy, a capacitor, a discharge circuit connecting saidcapacitor to said electrode and workpiece, a charging circuit havingcurrent-limiting properties that are independent of the spark gap andconnecting said capacitor to said energy source, and means for causing,at the instant of gap breakdown, the voltage across the work dischargearea to decrease to a Level below the spark extinction voltage of thespark gap work area in a period of time which is shorter than the timeperiod corresponding to the time constant of the discharge circuitconstituted by the capacitor and the spark gap.
 2. A device as claimedin claim 1 wherein said energy source comprises a source of constantcurrent, means responsive to the capacitor voltage for interrupting thesupply of current to the capacitor at a given voltage across thecapacitor, and a charging circuit for the gap work area including aninductor that forms a resonant circuit therewith.
 3. A device as claimedin claim 1 further comprising, a first active control element connectedbetween the energy source and the capacitor, a second active controlelement connected in parallel with the energy source, a first rectifierarranged between a control electrode of the first active element and thecapacitor, a second rectifier connected in series with the second activeelement and connected to the control electrode of the first activeelement, and means for deriving a control voltage for the second activeelement from the voltage across the capacitor.
 4. A device as claimed inclaim 1 further comprising, control means connected between the energysource and the capacitor, an amplifying element which forms part of thetransmission path between the energy source and the capacitor, means forcoupling the output of said amplifying element to said control means tocontrol the condition thereof, and means for coupling the voltage acrossthe capacitor to the control electrode of the amplifying element so asto produce a cutoff condition in said means thereby to cut off thevoltage from the source when the voltage across the capacitor decreases.5. A device as claimed in claim 4 wherein said control means includes acathode follower device connected in the transmission path and having acathode resistor, and means connecting the amplifying element inparallel with the cathode resistor of said cathode follower device.
 6. Adevice as claimed in claim 1 further comprising means coupled to saidcapacitor and responsive to the capacitor voltage for interrupting thesupply of charging current to the capacitor when the capacitor ischarged up to the breakdown voltage of the gap work area.
 7. A device asclaimed in claim 1 wherein said energy source and said charge circuittogether from a source of constant current for said capacitor and thespark gap work area which limits the current to a constant value duringthe whole spark erosion process.
 8. A device as claimed in claim 1further comprising, an inductor connected in series with the gap workarea, and means connecting the series combination of the inductor andthe gap work area in parallel with said capacitor across the energysource.
 9. Spark erosion apparatus for removing material from aworkpiece comprising, an electrode confronting said workpiece to definea spark discharge gap, a source of electric energy, a capacitor, acharge circuit having current-limiting properties that are independentof the spark gap, means including said charge circuit for coupling saidcapacitor and said spark gap to said energy source, a discharge circuitconnecting said capacitor to said electrode and workpiece, and meanscontrolled by the capacitor voltage for periodically interrupting theflow of charge current to said capacitor whenever the gap voltagereaches the gap breakdown voltage.
 10. Apparatus as claimed in claim 9further comprising a second capacitor directly connected across saidelectrode and workpiece, said second capacitor and said spark gap beingchosen to have a discharge time constant that is shorter than thedischarge time constant of the first capacitor and said spark gap. 11.Apparatus as claimed in claim 10 further comprising an inductorconnected in the charge circuit of said second capacitor and in thedischarge circuit of the first capacitor.
 12. Apparatus as claimed inclaim 11 wherein said energy source comprises a sourCe of constantcurrent, and said interrupting means comprises a control elementserially connected in the charge circuits of said first and secondcapacitors.
 13. Apparatus as claimed in claim 9 further comprising anonsaturating inductor connected in the discharge circuit of saidcapacitor for limiting the flow of discharge current from the capacitorto the spark gap during the breakdown periods of the gap.
 14. Apparatusas claimed in claim 9 wherein said interrupting means comprises acontrol element serially connected in the charge circuits of saidcapacitor and the spark discharge gap.
 15. Apparatus as claimed in claim9 wherein said energy source and said charge circuit together form asource of constant current for said capacitor and the spark dischargegap.